The present invention relates to a digital position control system, and more particularly to a system for head-positioning control, for use in a disk drive such as a hard disk drive.
Controls in a disk drive such as a hard disk drive (HDD) have become complicated. Further it is demanded that the controls be effected with high efficiency. Most of the disk drives recently developed can perform head-positioning control (servo control) by using a digital position control system that has a microprocessor (CPU).
The digital position control system monitors the condition of an object of control at prescribed intervals and executes control rules described in a program, thereby supplying a control signal to the object. Capable of executing high-level control rules, the digital position control system can help design a head-positioning control system of high precision.
A digital position control system is known which performs dead beat control (also called xe2x80x9cfinite time settling controlxe2x80x9d). Dead beat control is to move an object (e.g., a head) from its start position to a target position within a predetermined time. More specifically, the control signal is changed a prescribed number of times, at the intervals of monitoring the position of the object, and thereafter the settling control is performed until the object reaches a target state from the initial state within the prescribed time. The xe2x80x9cprescribed number of timesxe2x80x9d has been calculated from the initial state and target state of the object and corresponds to the order of the object.
The dead beat control is, however, disadvantageous in that the control signal greatly changes within a short time as it is repeatedly changed many times. Consequently, the object is likely to vibrate in a higher-order mode. That is, the dead beat control is much influenced by disturbance and higher-order mode. It is therefore an impractical method of controlling the positioning of the head of an HDD.
A digital position control system has been proposed which is a combination of dead beat control and model following control. As shown in FIG. 6, this control system comprises a model following control section 23 and a dead beat control section 24. The dead beat control section 24 generates a reference control value. A model 26 is a mathematical formula, or the equation of motion, which represents the dynamic characteristics of a plant 25 (i.e., object). It is assumed that the model 26 can be all state quantities of the plant 25. In practice, the plant 25 is a unit composed of an actuator and voice coil motor, which move the head of an HDD. The target position 21 and the start position 22, both shown in FIG. 6, are positions the head may take over the disk (data storage medium) incorporated in the HDD. The digital position control system shown in FIG. 6 is actually a system for controlling the positioning of the head in the HDD. It moves the head from the start position 21 to the target position 2.
The model following control section 23 calculates a control value from the response the model 26 has made to the reference control value. The response from the model 26 is output data representing the reference position of the model 26 and is, therefore, used as reference profile. The control value, thus calculated, is supplied to the plant 25, causing the plant 25 to operate in accordance with the reference profile. That is, the section 23 calculates a control value adjusted by the reference value generated by the dead beat control section 24, and the control value makes the output of the plant 25 (i.e., the position of the head) coincide with the output of the model 26 (i.e., the reference position for the head).
The system shown in FIG. 6, i.e., a combination of dead beat control and model following control, can control the positioning of the head at high speed and with high reliability. In the HDD, however, the actual gain (i.e., output-to-input ratio) may greatly differ from the design value due to the component differences and the changes in environmental conditions. More specifically, when the ambient temperature of the head changes greatly, the magnet in the voice coil motor (i.e., the drive motor for the actuator) may change in its characteristic.
When the gain of the system, more particularly the gain of the dead beat control section 24, changes, the seek profile for moving the head to the target position becomes unstable. The gain of the system usually decreases as the ambient temperature of the head rises. As the system gain decreases, the seek profile may become unstable. If this happens, it will take much time to settle the head, though the head can reach the target position within a relatively short time. The target position is actually one of the tracks formed on the disk, and the system must settle the head over this track. If the settle region in the seek profile changes greatly, the seek time (i.e., the time the head needs to reach the target position) will inevitably increase.
The object of the present invention is to provide a head-positioning control system in which a gain optimal to compensate for the characteristic changes of a plant can be set to perform head-positioning control with high reliability and at high speed.
The head-positioning control system according to the invention estimate the gain on the basis of the seek profile when the head is moved to the target position, calculates an optimal gain from the gain estimated, and sets the optimal gain thus calculated.
The system according to the invention is designed for used in a disk drive incorporating a head for reading and writing data on and from a disk and actuator means for moving the head from a start position to a target position on the disk. The system comprises: control means for controlling the actuator means in accordance with a gain and a difference between the position of the head and the target position, causing the actuator means to move the head from the start position to the target position; data-generating means for generating seek profile data representing a motion of the head from the start position to the target position; gain-estimating means for estimating the gain from the seek profile data generated by the data-generating means; and gain-setting means for determining a gain-adjusting value from the gain estimated by the gate-estimating means and setting the gain adjusted by the gain-adjusting value as a gain appropriate for the system.
In the head-positioning control system, an optimal gain can be set even if the gain initially set happens to change. The system can therefore accomplish head-positioning control with high reliability and at high speed. It performs the head-positioning control reliably and fast, particularly when it is a combination of a model following control section and a dead beat control section.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.